The invention refers to a switchable POTS splitter within a central office for separating a voice signal from a data signal and in particular to a POTS splitter for an asymmetric digital subscriber line (ADSL) architecture.
DSL (digital subscriber line) is a communication technology with underlies ISDN (integrated services digital network) and several variance that make up the x-DSL family of services. These include ADSL (asymmetric DSL), RADSL (rate adaptive DSL), SDSL (symmetric DSL) and VDSL (very high speed DSL).
xDSL provides a digital connection for customers or other end users. By using signaling methods both voice and data can be transmitted over telephone lines wherein the data transmission speed is higher than with ordinary modems.
The DSL technology provides digital access over ordinary telephone lines. ADSL is described in the ANSI T1.413 specification. ADSL supports the analog voice service which is also called plain old telephone service (POTS).
FIG. 1 shows a central office line card within an ADSL system architecture according to the state of the art. A remote transmission unit of a user comprises a POTS splitter for separating a voice signal from a data signal. The POTS splitter of the remote transmission unit is connected to a telephone and to a data terminal such as a personal computer. As can be seen from FIG. 1 the remote transmission unit is connected via a tip-line and a ring-line to the central office line-card within a central office. The central office line-card comprises although a POTS splitter having a low pass filter and a high pass filter. The low pass filter is connected via a POTS-signal-path to a digital telephone network and the high pass filter of the POTS splitter is connected via an ADSL-data-modem to a digital data network. The POTS splitter separates the voice signal having a low frequency from the data signal which has a high frequency.
For establishing a data communication between a first user connected to a digital telephone network and the remove transmission unit of a second user as shown in FIG. 1 the first user dials the telephone number of the second user which is received by a central office line-card within a local central office to which the first user is connected. The first central office line-card detects the incoming call of the first user and signals this call demand to the central office line-card within the local central office of the second user as shown in FIG. 1. The incoming command will e.g. be detected by the CODEC device within the central office line-card which generates a switch control signal for an mechanical relay. The relay switches off the POTS signal path and connects the ring signal generator via a current measuring resistor RM to the low pass filter within the POTS splitter of the central office line card. The ring signal generator generates a ringing signal having the frequency of approximately 20 to 50 Hz and a high voltage amplitude e.g. 120V (85V rms). This low frequency ringing signal is passed by the low pass filter to the ring line and transmitted via the ring line to the POTS splitter device of the remote transmission unit. The ringing signal received by the remote transmission unit is filtered by the POTS splitter of the remote transmission unit and passed to the telephone of the second user. In case that the second user picks tip the telephone the DC-current flows through the ring line and is detected by the measuring resistor RM. To establish the communication between the two users the relay is then switched back to close the POTS signal path between the subscriber line interface circuit (SLIC) and the low pass filter of the POTS splitter. During the telephone call i.e. the voice data exchange it is possible for the second user to exchange information data via the ADSL-data-modem and the digital data network simultaneously. This is possible because the voice signal and the data signal use different frequency bands.
The relay in the central office line card as shown in FIG. 1 has several disadvantages. A relay has a considerable size and cannot be integrated. A further disadvantage is that the electromechanical relays operate with magnetic fields. These magnetic fields cannot be confined to the relay so that interaction of magnetic fields between adjacent electromagnetic components must be accounted for in the printed circuit bord design. Electromagnetic relays generate electrical and audible noise that can be problematic in printed circuit bord applications. Magnetic relays contain moving parts and are consequently sensitive to physical shock and vibration. A further major disadvantage of electromechanical relays is contact bouncing during the change from one switching position to the other switching position which leads to mechanical aberrations a lower reliability and life time of the mechanical relay. Since electromagnetic relays must energize a coil before switching can take place they have high power consumption and generate a considerable amount of heat.
To overcome the above mentioned disadvantages the relay has been replaced in a central office line card according to the state of the art by an electronic switch. FIG. 2 shows a central office line card according to the state of the art comprising an electronic switch instead of a relay. The CODEC-circuit detects the incoming call of a user and controls the electronic switch through a switch control signal. When the CODEC-circuit detects the incoming call command switches S11 and S21 are closed so that the ringing signal generated by the ring signal generator is applied to the low pass filter. To interrupt the POTS signal path the switches S12, S22 within the electronic switch are switched off. When the measuring resistor RM measures the picking up of the telephone by means of the flowing DC current the switches S11, S21 are switched off and the corresponding switches S12, S22 are switched on to close the POTS signal path and to establish the voice-communication between the two users.
The low pass filter of POTS-splitter in the central office line card as shown in FIG. 2 is a passive low pass filter having two coils L. The low pass filter has a low filter order so that the SLIC and CODEC-circuit are able so synthesize the required terminating resistance or terminating impedance for different countries. The required terminating impedances for different countries vary, so that for instance the terminating impedance in the United States is different from the terminating impedance in Germany. The synthesized terminating impedance to be accomplished so that the standard for POTS-application e.g. the return loss is not violated. A low pass filter with a low filter order has further advantages in that it is smaller and easier to integrate.
The ring signal generator generates a ringing signal with a high voltage amplitude. Consequently the switch S11 has to sustain such a high voltage and is formed normally by a thyristor device. When the thyristor switch S11 is switched on and the ringing signal generated by the ringing signal generator is applied to the low pass filter of the POTS-splitter the impedance changes because the impedance of the thyristor switch S11 varies when the ringing signal is switched through the thyristor has a high impedance during zero crossing of the ringing signal.
The thyristor switch S11 has a non linear switching behavior. For signals greater e.g. than 1 Volt the switching resistance of the thyristor switch S11 is less than 10xcfx89 and for signals having an amplitude below 1 Volt the resistance increases to several kilo ohms. The non linearity as well as the impedance changes of the switch S11 in the central office line card according to the state of the art as shown in FIG. 2 disturb the ADSL data transmission through the POTS splitter. This in return leads to non correctable errors in the data transmission.
Accordingly it is an object of the present invention to provide a POTS-splitter for a central office wherein the ringing signal does not affect the simultaneous data transmission.
This object is achieved by a switchable POTS-splitter having the features of rain claim 1.
The invention provides a switchable POTS-splitter for separating a voice signal from a data signal having
a high pass filter for the data signal and
a low pass filter for the voice signal and for a ringing signal which is generated by v ringing signal generator and switched by means of a signal feeding switch to the low pass filter for transmission to a remote transmission unit via a ring line,
wherein the low pass filter comprises at least one capacitor and a capacitor switch for switching said capacitor to the ring line when the ringing signal is transmitted.
In a preferred embodiment of the switchable POTS-splitter according to the present invention said capacitor switch is an electronic switch controlled by a CODEC-circuit.
This has the advantage that an electronic switch is easy to integrate and can be controlled by a control circuit without additional driver circuits.
In a further preferred embodiment of the swichable POTS-splitter according to the present invention said signal feeding switch is an electronic switch-controlled by a CODEC-circuit.
The signal feeding switch is a thyristor device in a preferred embodiment of the switchable POTS-splitter according to the present invention.
In a further preferred embodiment of the switchable POTS-splitter according to the present invention the low pass filter is a passive filter of low order.
This has the advantage, that the POTS-splitter is operable when there is a power failure.
In a further preferred embodiment of the POTS-splitter according to the present invention the low pass filter includes at least one coil having an inductivety of around 5 mH.
In a further preferred embodiment of the switchable POTS-splitter according to the present invention the at least one capacitor has a capacity of around 100 nF.
The high pass filter is also a passive filter in a preferred embodiment of the POTS-splitter according to the present invention.
The high pass filter is preferably connected to an ADSL-data-modem.
The remote transmission unit is preferably an ADSL-transmission unit having a separate POTS-splitter.